Adhesive for silicone rubber

ABSTRACT

An adhesive for silicone rubber, comprising at least the following: A 100 parts by weight of an organopolysiloxane having an average of two or more alkenyl groups per molecule; B an organopolysiloxane having an average of two or more silicon bonded hydrogen atoms in each molecule, used in an amount such that the molar ratio of silicon bonded hydrogen atoms in component B to alkenyl groups in component A at which the molar ratio of the silicon bonded hydrogen atoms in this component to the alkenyl groups in component A is from 0.01 to 20 (i.e. from 1:100 to 20:1); C from 5 to 200 parts by weight of calcium carbonate powder with a BET specific surface area of from 5 to 50 m 2 g; and D a platinum-based catalyst, used in an amount capable of inducing curing in the present composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

[0003] Not applicable.

FIELD OF INVENTION

[0004] The present invention relates to an adhesive for silicone rubberthat can be readily bonded to silicone rubber materials.

BACKGROUND OF THE INVENTION

[0005] Silicone rubber materials are well known as having excellentwater repellency, weatherability, heat resistance, and the likeproperties. Liquid silicone rubbers are used as coating agents orfilm-forming agents on a variety of substrates. However, these siliconerubbers have poor adhesive properties, and are therefore adapted for usewith silicone rubber adhesives. Japanese patent application publicationKokai No. 61-278580, equivalent to U.S. Pat. No. 4,766,193, describes anumber of adhesives used with liquid silicone rubbers which compriseorganic peroxides, condensation catalysts and organopolysiloxanes havingalkenyl groups and hydroxyl or alkoxy groups bonded to silicon atoms.

[0006] Japanese patent application publication Kokai 62-90369,equivalent to U.S. Pat. No. 4,889,576, discusses a method of adheringsilicone-coated pieces of fabric together by overlapping siliconerubber-coated fabrics and inserting into the overlap region a siliconerubber adhesive. The silicone rubber adhesive is either an additioncuring composition containing a platinum-type catalyst or aradical-curing composition containing an organoperoxide type catalyst.The overlap region containing the silicone rubber adhesive is hot-curedduring or after press-adhering. In particular, Japanese patentapplication publication Kokai 62-90369, equivalent to U.S. Pat. No.4,889,576, discloses a silicone composition that is cured by an additionreaction, but this composition still has inadequate adhesion.

[0007] Japanese patent application publication Kokai 10-60281 disclosesa silicone composition that contains calcium carbonate powder surfacetreated with a partially hydrolysed condensate of a tetraalkoxysilane.The silicone rubber composition, containing the calcium carbonate fillertreated with a hydrolysed condensate of tetraalkoxysilanes, provides alow evolution of a hydrogen gas during its storage, and has superiorfluidity and electrical characteristics and is said to prevent foamingduring the storage of the composition. There is no teaching in thisdocument that the composition can be used as an adhesive for siliconerubber, and the adhesion of this composition to silicone rubber is stillinadequate.

[0008] As a result of research aimed at overcoming the aforementionedshortcomings, the inventors have developed the present invention basedon the fact that adequate adhesion to silicone rubber can be obtained byutilising an addition reaction curable silicone composition containingcalcium carbonate powder with a prescribed BET specific surface area.

[0009] Specifically, an object of the present invention is to provide anadhesive for silicone rubber that has good adhesion to silicone rubbermaterials.

DETAILED DESCRIPTION OF THE INVENTION

[0010] In accordance with the present invention there is provided asilicone rubber adhesive composition comprising the followingcomponents:

[0011] A. 100 parts by weight of an organopolysiloxane having an averageof two or more alkenyl groups per molecule;

[0012] B. an organopolysiloxane having an average of two or more siliconbonded hydrogen atoms in each molecule, in an amount such that the molarratio of silicon bonded hydrogen atoms in component B to alkenyl groupsin component A is from 0.01 to 20, i.e. from 1:100 to 20:1;

[0013] C. From 5 to 200 parts by weight of a calcium carbonate powderselected from the group of untreated calcium carbonate, calciumcarbonate treated with an organic acid and calcium carbonate treatedwith an ester of an organic acid, said calcium carbonate having a BETspecific surface area of from 5 to 50 m²/g; and

[0014] D. a platinum-based catalyst, in an amount sufficient to effectcuring of the composition.

[0015] The organopolysiloxane of component A is the principal ingredientof the adhesive. This component is characterized by having an average oftwo or more alkenyl groups per molecule. Examples of such alkenyl groupsinclude vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl groups,of which the vinyl group is preferred. Examples of organic groups bondedto silicon atoms other than the alkenyl groups in component A includemethyl, ethyl, propyl, butyl, pentyl, hexyl, and other alkyl groups;phenyl, tolyl, xylyl, and other aryl groups; and 3-chloropropyl,3,3,3-trifluoropropyl, and other alkyl halide groups, of which groupsthe methyl group is preferred. Component A may have a molecularstructure that is linear, partially branched linear, branched,reticulated, or dendritic. The viscosity of component A at 25° C., ispreferably from 100 to 1,000,000 mPa·s, and most preferably from 100 to500,000 mPa·s.

[0016] Examples of organopolysiloxanes which may be utilised ascomponent A include polydimethylsiloxanes in which both ends of themolecular chain are blocked by dimethylvinylsiloxy terminal groups;dimethylsiloxane/methylvinylsiloxane copolymers in which both ends ofthe molecular chain are blocked by dimethylvinylsiloxy terminal groups;dimethylsiloxane/methylvinylsiloxane copolymers in which both ends ofthe molecular chain are blocked by trimethylsiloxy terminal groups;organopolysiloxanes comprising siloxane units expressed by the formula(CH₃)₃SiO_(1/2), siloxane units expressed by the formula(CH₃)₂(CH₂═CH)SiO_(1/2), and siloxane units expressed by the formulaSio_(4/2); organopolysiloxanes obtained by methods in which some or allof the methyl groups of these organopolysiloxanes are substituted byalkyl groups such as ethyl, propyl, and the like, aryl groups, such asphenyl, tolyl, and the like, or alkyl halide groups such as3,3,3-trifluoropropyl or the like; organopolysiloxanes obtained bymethods in which some or all of the vinyl groups of theseorganopolysiloxanes are substituted by allyl, propenyl, or other alkenylgroups; and mixtures of two or more of these organopolysiloxanes.

[0017] Component B is an organopolysiloxane which functions as thecross-linking agent of the adhesive of the present invention. ComponentB is characterized by having an average of two or more silicon bondedhydrogen atoms (i.e. Si—H bonds) per molecule. The silicon bondedhydrogen atoms may be anywhere on the organopolysiloxane that formscomponent B, for example they may be on silicon atoms forming part of aterminal group at the end of the organopolysiloxane chain or on siliconatoms in non-terminal groups such as silicon atoms in the backbone ofthe organopolysiloxane polymer chain or, when present, in side chains ofthe organopolysiloxane polymer. The organic groups bonded to the siliconatoms in component B may include methyl, ethyl, propyl, butyl, pentyl,hexyl, and other alkyl groups; phenyl, tolyl, xylyl, and other arylgroups; and 3-chloropropyl, 3,3,3-trifluoropropyl, and other alkylhalide groups, of which the methyl group is preferred. Component B mayhave a linear, partially branched linear, branched, reticulated, ordendritic molecular structure. The viscosity of component B at 25° C.,is preferably from 1 to 1,000,000 mPa·s, but most preferably from 1 to10,000 mPa·s.

[0018] Examples of organopolysiloxanes which may be utilised asComponent B include polydimethylsiloxanes in which both ends of themolecular chain are blocked by dimethyl hydrogen siloxy terminal groups;polymethyl hydrogen siloxanes in which both ends of the molecular chainare blocked by trimethylsiloxy terminal groups; dimethylsiloxane/methylhydrogen siloxane copolymers in which both ends of the molecular chainare blocked by trimethylsiloxy terminal groups; cyclic polymethylhydrogen siloxanes; organopolysiloxanes comprising siloxane unitsexpressed by the formula (CH₃)₂HSiO_(1/2) and siloxane units expressedby the formula SiO_(4/2) Component B may also be an organopolysiloxaneas described above in which some or all of the methyl groups have beenreplaced by alternative alkyl groups such as ethyl, propyl, isopropyland the like, aryl groups such as phenyl and tolyl, and the like, oralkyl halide groups such as 3,3,3-trifluoropropyl or the like. ComponentB may, in a further alternative be a mixture of two or more of the aboveorganopolysiloxanes. In the case where Component B comprises a mixtureof polymers having Silicon bonded hydrogen atoms, it is preferred thatthe mixture comprises:

[0019] i. an organopolysiloxane having silicon bonded hydrogen atomssolely in terminal groups of the molecular chain; and

[0020] ii. an organopolysiloxane having silicon bonded hydrogen atoms innon terminal groups.

[0021] Mixtures of this type are preferred because of the resultingmechanical characteristics, particularly improved elongation, of theresulting cured adhesive product.

[0022] The molar ratio of silicon bonded hydrogen atoms in component Bto alkenyl groups in Component A is from 0.01 to 20, i.e. from 1:100 to20:1, preferably from 0.1 to 10, i.e. from 1:10 to 10:1, and mostpreferably from 0.1 to 5, i.e. from 1:10 to 5:1. This is because theresulting cured adhesive tends to be inadequately cross-linked, andthereby is inadequately cured, if the content of component B is belowthe aforementioned range, and the mechanical characteristics of theresulting cured product tend to be adversely affected if the contentexceeds the aforementioned range.

[0023] When component B is a mixture of:

[0024] i. an organopolysiloxane having silicon bonded hydrogen atomssolely in terminal groups of the molecular chain; and

[0025] ii. an organopolysiloxane having silicon bonded hydrogen atoms innon terminal groups.

[0026] the molar ratio of silicon bonded hydrogen atoms inorganopolysiloxane i to alkenyl groups in component A is preferably from0.01 to 10, i.e. from 1:100 to 10:1, more preferably from 0.1 to 10,i.e. from 1:10 to 10:1 and most preferably from 0.1 to 5, i.e. from 1:10to 5:1 and the molar ratio of silicon bonded hydrogen atoms inorganopolysiloxane ii to alkenyl groups in component A is preferablyfrom 0.5 to 20,i.e. from 1:2 to 20:1, more preferably from 0.5 to 10,i.e. from 1:2 to 10:1, and most preferably from 0.5 to 5, i.e. from 1:2to 5:1. However it is essential that when Component B comprises amixture of organopolysiloxanes such as organopolysiloxanes i and iiabove, the total combined molar ratio of silicon bonded hydrogen atomsin the organopolysiloxanes making up the mixture of component B, e.g.organopolysiloxanes i and ii, to alkenyl groups in component A is withinthe range of from 0.01 to 20, i.e. from 1:100 to 20:1.

[0027] Component C is a calcium carbonate powder, as hereinbeforedescribed, utilised in the present invention to improve the adhesion ofthe adhesive of the present invention to silicone rubber. The calciumcarbonate powder is characterized as having a BET specific surface areaof from 5 to 50 m²/g, and preferably from 10 to 50 m²/g. Examples ofcalcium carbonate powders suitable for use as component C in the presentinvention include untreated dry-ground calcium carbonate powders, whichare also referred to as heavy calcium carbonates, untreated precipitatedcalcium carbonates, which are also known as light calcium carbonate, andpowders obtained by treating the surfaces of these calcium carbonatepowders with organic acids or their esters. Preferred organic acids arefatty acids, such as stearic acid and resin acids. Precipitated calciumcarbonate powders are preferred, with precipitated calcium carbonatepowders surface-treated with an organic acid such as a fatty acid orresin acid particularly preferred.

[0028] The content of component C in the adhesive of the presentinvention is from 5 to 200 parts by weight, and preferably from 10 to100 parts by weight, per 100 parts by weight of component A. This isbecause the adhesion of the adhesive of the present invention tosilicone rubber tends to decrease if the content of component C is belowthe aforementioned range, and a uniform adhesive is difficult to prepareif the content exceeds the aforementioned range.

[0029] Component D is a platinum-based catalyst for promoting the curingof the adhesive of the present invention. Examples of component Dinclude fine platinum powder, platinum black, chloroplatinic acid,platinum tetrachloride, alcoholic solutions of chloroplatinic acid,platinum olefin complexes, platinum alkenyl siloxane complexes, platinumcarbonyl complexes, and fine powders obtained by the dispersion of anyof the above platinum-based catalysts in methyl methacrylate resins,polycarbonate resins, polystyrene resins, silicone resins, and otherthermoplastic organic resins.

[0030] Component D of the adhesive of the present invention must bepresent in an amount sufficient to ensure that the adhesive compositionwill cure. Preferably, however component D is present in the adhesive ofthe present invention in an amount of from 0.01 to 500 parts by weight,and preferably from 0.1 to 100 parts by weight, per 1,000,000 parts byweight of component A when expressed as the content of platinum metal incomponent D.

[0031] Optionally a Component E may be added into the adhesivecomposition. Component E is a silica powder which is utilised to improvethe mechanical strength of the resulting cured adhesive. Examples ofcomponent E include fumed silica, precipitated silica, baked silica,pulverized quartz, and powders obtained by treating the surfaces of theabove silica powders with organosilicon compounds such asorganoalkoxysilanes, organohalosilanes and organosilazanes. A silicapowder with a BET specific surface area of at least 50 m²/g ispreferably used as component E in order to adequately improve themechanical strength of the resulting cured adhesive.

[0032] Whilst Component E is an optional ingredient, the adhesive of thepresent invention may comprise any appropriate amount of Component E butpreferably comprises from 1 to 100 parts by weight, most preferably from1 to 50 parts by weight, per 100 parts by weight of component A.Component E may be incorporated in the adhesive of the present inventionto improve the mechanical strength of the resulting cured adhesive.

[0033] The following optional components may also be added to theadhesive of the present invention: fumed titanium oxide, carbon black,diatomaceous earth, iron oxide, aluminium oxide, aluminosilicates,calcium carbonate, zinc oxide, aluminium hydroxide, silver, nickel, andother inorganic fillers, as well as fillers obtained by treating thesurfaces of these fillers with the aforementioned organosiliconcompounds.

[0034] The following tackifiers may also be added to the adhesive of thepresent invention in order to enhance its adhesiveness: methyltrimethoxysilane, vinyl trimethoxysilane, allyl trimethoxysilane,3-methacryloxypropyl trimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-aminopropyl trimethoxysilane,N-(2-aminoethyl)-3-aminopropyl trimethoxysilane,bis(trimethoxysilyl)propane, bis(trimethoxysilyl)hexane, and othersilane coupling agents; tetraethyl titanate, tetrapropyl titanate,tetrabutyl titanate, tetra(2-ethylhexyl)titanate, titanium ethylacetonate, titanium acetylacetonate, and other titanium compounds; ethylacetoacetate aluminium diisopropylate, aluminium tri(ethylacetoacetate), alkyl acetoacetate aluminium diisopropylates, aluminiumtris(acetoacetonate), aluminium monoacetyl acetonate bis(ethylacetoacetate), and other aluminium compounds; and zirconiumacetylacetonate, zirconium butoxyacetyl acetonate, zirconiumbisacetylacetonate, zirconium ethylacetoacetate, and other zirconiumcompounds. The content of these tackifiers, when present, while notsubject to limitations, should preferably be within a range of from 0.01to 10 parts by weight per 100 parts by weight of component A.

[0035] The adhesive of the present invention may further contain one ormore of the following curing inhibitors, which are used to enhance thestorage stability, handleability, and workability of the resultingadhesive. Examples of the curing inhibitors which may be utilised forthe present invention include 3-methyl-1butyn-3-ol,3,5-dimethyl-1-hexyn-3-ol, 3-phenyl-1-butyl-3-ol, and otheracetylene-based compounds; 3-methyl-3-penten-1-yn,3,5-dimethyl-3-hexen-1-yn, and other en/yn compounds;1,3,5,7-tetramethyl-1,3,5,7-tetravinyl cyclotetrasiloxane,1,3,5,7-tetramethyl-1,3,5,7-tetrahexenyl cyclotetrasiloxane, methylvinylsiloxanes blocked by silanol terminal groups at both ends of themolecular chain, copolymers of methylvinyl siloxanes anddimethylsiloxanes blocked by silanol terminal groups at both ends of themolecular chain, and other organosiloxane compounds containing vinylgroups in each molecule in an amount of 5 wt % or greater; benzotriazoleand other triazoles; and phosphines, mercaptans, hydrazines, and othercuring inhibitors. When used in the adhesive of the present inventionthese curing inhibitors are preferably present in an amount of from0.001 to 5 parts by weight per 100 parts by weight of component A.

[0036] Any suitable method may be used for the preparation of theadhesive in accordance with the present invention and as such theadhesive may be prepared by mixing components A to D and any of theoptional components as needed in any order. However, in the case whencomponent E is present in the adhesive of the present invention it ispreferred that components B, C and D are added during or after thepreparation of a base compound obtained by heating and mixing componentsA and E. When it is necessary to add one or more of the optionalcomponents, their addition may be made during the preparation of thebase compound. However, if the optional ingredients may decompose duringthe high temperature mixing stage of the base compound preparation,their addition should preferably be made together with the addition ofcomponents B to D. It is also possible to add the aforementionedorganosilicon compounds such as organoalkoxysilanes, organohalosilanesand organosilazanes to treat the surface of component E in situ when thebase compound is prepared. Twin rollers, kneader/mixers, roll mixers,and other commonly known kneading apparatus can be used in order toprepare the adhesive of the present invention.

[0037] In a further embodiment of the invention there is provided amethod of bonding silicone rubber to silicone rubber materials, saidmethod comprising:

[0038] (I) treating a surface of the silicone rubber, or treating thesurface of the silicone rubber material, or treating both the surface ofa silicone rubber and a silicone rubber material with the adhesivecomposition of the present invention;

[0039] (II) contacting the treated surface of the silicone rubber withan untreated silicone rubber material, or contacting the treated surfaceof the silicone rubber material with an untreated silicone rubbermaterial, or contacting the treated surface of the silicone rubber withthe treated surface of the silicone rubber material to form a composite,and,

[0040] (III) allowing the adhesive composition to cure.

EXAMPLES

[0041] The adhesive in accordance with the invention will now bedescribed in further detail through working examples. All viscosityvalues provided in the examples were measured at 25° C.

Working Example 1

[0042] 100 parts by weight of a polydimethylsiloxane blocked bydimethylvinylsiloxy terminal groups at both ends of the molecular chainhaving a viscosity of 40,000 mPa·s, 10 parts by weight of fumed silicahaving a BET specific surface area of 200 m²/g, 1.5 parts by weight of ahexamethyldisilazane surface-treated with silica, and 1 part by weightof water were uniformly mixed and then heated/mixed for 2 hours at atemperature of 170° C. under vacuum, to yield a base compound.

[0043] The following components were then admixed into 110 parts byweight of the base compound; 40 parts by weight of a precipitatedcalcium carbonate powder, Hakuenka CCR from Shiraishi K K,surface-treated with a fatty acid and provided with a mean grain size of0.12 μm and a BET specific surface area of 18 m²/g; apolydimethylsiloxane blocked by dimethyl hydrogen siloxy terminal groupsat both ends of the molecular chain having a viscosity of 10 mPa·s in anamount such that the molar ratio of silicon bonded hydrogen atoms incomponent B to vinyl groups in component A corresponds to a value of0.2, i.e. 1:5; a dimethylsiloxane/methyl hydrogen siloxane copolymerhaving a viscosity of 6 mPa·s, an average of three silicon bondedhydrogen atoms in each molecule, and trimethylsiloxy blocking groups atboth ends of the molecular chain, in an amount such that the molar ratioof silicon bonded hydrogen atoms in component B to vinyl groups incomponent A corresponds to a value of 2.8 i.e. 2.8:1; adimethylsiloxane/methylvinylsiloxane copolymer, 0.2 parts by weight;content of vinyl groups: 10 wt %, blocked by silanol terminal groups atboth ends of the molecular chain, provided with a viscosity of 40 mPa·s,and used as a curing inhibitor; 0.5 parts by weight of titaniumtetrabutoxide; and a platinum 1,3-divinyl tetramethyldisiloxane complex,used in an amount of 30 parts by weight in terms of the platinum metalin the catalyst per 1,000,000 parts by weight of thepolydimethylsiloxane in the base compound. The resulting adhesive wascured by being allowed to stand for 7 days at 25° C.

[0044] The hardness of the cured adhesive was measured with a Type Adurometer in accordance with JIS K 6253.

[0045] The tensile strength and elongation of the cured adhesive wasmeasured by the method described in JIS K 6251, using a dumbbell-shapedNo. 3 test piece was fabricated according to JIS K 6251. The measurementresults are shown in Table 1.

[0046] The adhesion of the adhesive to silicone rubber was measured asdescribed below in accordance with the method detailed in JIS K 6854.The measurement results are shown in Table 1.

[0047] Two nylon tapes coated with silicone rubber having a width of 50mm were laminated together such that the thickness of the adhesive was0.5 mm, and the adhesive was cured by being allowed to stand for 7 daysat 25° C. The resulting laminated tape was subjected to T-type peelingtests at rate of 200 mm/min.

Working Example 2

[0048] An adhesive for silicone rubber was prepared in the same manneras in Working Example 1 except that the polydimethylsiloxane, which wasblocked by dimethyl hydrogen siloxy terminal groups at both ends of themolecular chain and had a viscosity of 10 mPa·s, and thedimethylsiloxane/methyl hydrogen siloxane copolymer which had aviscosity of 6 mPa·s, an average of three silicon bonded hydrogen atomsin each molecule, and trimethylsiloxy blocking groups at both ends ofthe molecular chain, used in Working Example 1 were replaced with adimethylsiloxane/methyl hydrogen siloxane copolymer. Thisdimethylsiloxane/methyl hydrogen siloxane copolymer had a viscosity of 6mPa·s, contained an average of three silicon bonded hydrogen atoms ineach molecule, possessed trimethylsiloxy blocking groups at both ends ofthe molecular chain, and was used in an amount such that the molar ratioof silicon bonded hydrogen atoms in component B to vinyl groups incomponent A corresponds to a value of 3.0. The physical properties andadhesive power of the cured adhesive were measured in the same manner asin Working Example 1. The results are shown in Table 1.

Working Example 3

[0049] An adhesive for silicone rubber was prepared in the same manneras in Working Example 1 except that the precipitated calcium carbonatepowder with a mean grain size of 0.12 μm and a BET specific surface areaof 18 m²/g used in Working Example 1 was replaced with a precipitatedcalcium carbonate powder (MT-100 from Maruo Calcium) surface-treatedwith a resin acid and fashioned to have a mean grain size of 0.07 μm anda BET specific surface area of 17 m²/g. The physical properties andadhesive power of the cured adhesive were measured in the same manner asin Working Example 1. The results are shown in Table 1.

Comparative Example 1

[0050] An adhesive for silicone rubber was prepared in the same manneras in Working Example 1 except that the precipitated calcium carbonatepowder with a mean grain size of 0.12 μm and a BET specific surface areaof 18 m 2/g used in Working Example 1 was replaced with a dry-groundcalcium carbonate powder (P-30 from Toyo Fine Chemical) having a meangrain size of 0.68 μm and a BET specific surface area of 3.4 m²/g. Thephysical properties and adhesive power of the cured adhesive weremeasured in the same manner as in Working Example 1. The results areshown in Table 1.

Comparative Example 2

[0051] An adhesive for silicone rubber was prepared in the same manneras in Working Example 1 except that the precipitated calcium carbonatepowder with a mean grain size of 0.12 μm and a BET specific surface areaof 18 m^(2/)g used in Working Example 1 was replaced with a pulverizedquartz powder (Crystallite VXS2 from Tatsumori) having a mean grain sizeof 5 μm and a BET specific surface area of 3.4 m²/g. The physicalproperties and adhesive power of the cured adhesive were measured in thesame manner as in Working Example 1. The results are shown in Table 1.TABLE 1 Example Compar- Compar- Working Working Working ative ativeExample Example Example Example Example Parameter 1 2 3 1 2 Hardness 1010 9 9 8 Tensile 3.5 3.9 3.2 2.2 2.0 strength (MPa) Elongation 1500 13751450 1050 1100 (%) Adhesive 23 20 20 12 11 power (kgf/50 mm)

[0052] The inventive adhesive for silicone rubber is characterized byhaving good adhesion to silicone rubber materials.

1. A silicone rubber adhesive composition comprising the followingcomponents: A. 100 parts by weight of an organopolysiloxane having anaverage of two or more alkenyl groups per molecule; B. anorganopolysiloxane having an average of two or more silicon bondedhydrogen atoms in each molecule, in an amount such that the molar ratioof silicon bonded hydrogen atoms in component B to alkenyl groups incomponent A is from 0.01 to 20; C. From 5 to 200 parts by weight of acalcium carbonate powder selected from the group of untreated calciumcarbonate, calcium carbonate treated with an organic acid and calciumcarbonate treated with an ester of an organic acid, said calciumcarbonate powder having a BET specific surface area of from 5 to 50m²/g; and D. a platinum-based catalyst, in an amount sufficient toeffect curing of the composition.
 2. A composition in accordance withclaim 1, in which component C is a precipitated calcium carbonatepowder.
 3. A composition in accordance with claim 1, in which there isadditionally provided a component E which is a silica powder in anamount of from 1 to 100 parts by weight per 100 parts by weight ofcomponent A.
 4. A composition in accordance with claim 1, in whichcomponent B is a mixture of: i. an organopolysiloxane having siliconbonded hydrogen atoms solely in terminal groups of the molecular chain;and ii. an organopolysiloxane having silicon bonded hydrogen atoms innon terminal groups.
 5. A composition in accordance with claim 1, inwhich the molar ratio of silicon bonded hydrogen atoms in component B toalkenyl groups in component A is from 0.1 to
 5. 6. A composition inaccordance with claim 1 in which the calcium carbonate is pre-treatedwith a fatty acid or a resin acid.
 7. A composition in accordance withclaim 3 wherein the silica powder has a BET specific surface area of atleast 50 m²/g.
 8. A method of bonding silicone rubber to silicone rubbermaterials, said method comprising: I. treating a surface of the siliconerubber, or treating the surface of the silicone rubber material, ortreating both the surface of a silicone rubber and a silicone rubbermaterial with the adhesive composition of claim 1; II. contacting thetreated surface of the silicone rubber with an untreated silicone rubbermaterial, or contacting the treated surface of the silicone rubbermaterial with an untreated silicone rubber material, or contacting thetreated surface of the silicone rubber with the treated surface of thesilicone rubber material to form a composite, and, III. allowing theadhesive composition to cure.
 9. A method of making a silicone rubberadhesive as claimed in claim 3 wherein components B, C, and D, are addedduring the preparation of a base compound obtained by heating and mixingcomponents A and E.
 10. A method of making a silicone rubber adhesive asclaimed in claim 3 wherein components B, C, and D, are added after thepreparation of a base compound prepared and obtained by heating andmixing components A and E.